Try this experiment: -
Take an empty plastic 4 pint (2.27L) milk container. This size is best because it has a good handle, and can be held safely.
Part fill it with hot tap water. Screw on the top and shake the container.
Q.1 What happens?
Q.2 Why?
I have tried this experiment with a little boiled water from the kettle, but care needs to be exercised to avoid spillage. Also, when you shake the container, point the top away from you. The same thing happens, but with more drama.
Lastly, hopefully this experiment will encourage folk to recycle plastics. I love flattening the sof container after emptying it. The plastic doesn't have a memory, and less space is taken up in the recycling bin.
I am sure this experiment could be carried out in a Lab. I am thinking of trying to use a set quantity of water at different temperatures, and to take physical measurements to compare what happens.
I won't post the answers with this, just in case your children or students read these forums. However, lets see what replies arrive here, and I will then give my layman's take on what happens and why. I'm sure the physicists and engineers will put me straight.